The present invention relates to a control valve for a variable displacement type compressor, and, more particularly, to a control valve for a variable displacement type compressor, which adjusts the displacement of the compressor in accordance with the pressure in a crank chamber.
Generally speaking, in a variable displacement type swash plate compressor for use in a vehicle air-conditioning system, the inclination angle of a swash plate, which is located in a crank chamber, is changed in accordance with the pressure in the crank chamber (crank pressure Pc). The crank chamber is connected to a suction chamber via a bleed passage. In the bleed passage is a displacement control valve, which performs feedback control of the displacement to keep the pressure in the vicinity of the outlet of an evaporator (suction pressure Ps), or the pressure of the refrigerant gas that is drawn in by the compressor (suction pressure Ps), at a target suction pressure even when the thermal load varies.
For example, Japanese Unexamined Patent Publication (KOKAI) No. Hei 6-26454 discloses a relief side control valve of a variable target suction pressure type compressor. The bleed passage connects the crank chamber of the compressor to a suction pressure area. Defined in the valve housing of the control valve is a valve chamber, which constitutes part of the bleed passage. Located in the valve chamber are a valve body and a bellows, which actuates the valve body in accordance with the suction pressure Ps. The degree of opening of the valve is adjusted in accordance with the expansion and constraction of the bellows. The control valve has a transmission rod and an electromagnetic actuator connected to the bellows via the valve body. The force of the electromagnetic actuator varies in accordance with the electric current supplied to the actuator. A target suction pressure Pset varies by controlling the magnitude of the electric urging force applied by the actuator.
FIG. 7 is a graph showing the relationship, which is simulated by a computer, between the suction pressure Ps and the crank pressure Pc when the displacement of the compressor is controlled by the aforementioned relief side control valve. Seven characteristic curves xcfx861 to xcfx867 indicate the characteristics of seven types of control valves, the conditions of which differ only in the aperture size of the valve hole. The characteristic curve xcfx861 corresponds to the control valve that has the smallest aperture size, and the characteristic curve xcfx867 corresponds to the control valve that has the largest aperture size. The aperture size increases as the number following xcfx86 increases. Each characteristic curve has a right portion rightward that extends from lower left to upper right. The asymptotic line of each curve is the diagonal line xcex1 of the graph (linear line of Pc=Ps). Each curve has left portion that extends from upper left to lower right and is continuous with the right portion, and a critical point (minimum point) occurs between the two portions of each curve.
The Pc/Ps gain is one index to evaluate the response characteristics of a control valve for a compressor. The Pc/Ps gain is scalar defined as the absolute value of the ratio of the amount of change xcex94Pc in the crank pressure Pc, which is a control output value, to the amount of change xcex94Ps in the suction pressure Ps, which is a control input value. In FIG. 7, the differential (dPc/dPs) of the left portion of each of the characteristic curves xcfx861-xcfx867, or the inclination of the associated tangential line, is equivalent to the Pc/Ps gain (xcex94Pc/xcex94Ps).
In general, the greater the gain is, the better the response characteristic of the control valve is. Therefore, a compressor that incorporates such a control valve can quickly and precisely respond to a change in the thermal load. The control valve that has a high gain causes the actual suction pressure Ps to quickly converge to near the target suction pressure Pset. The fluctuation of the actual suction pressure Ps is extremely small. In a control valve that has a small gain, by way of contrast, the actual suction pressure Ps does not converge to the target suction pressure Pset and significantly fluctuates up and down, which is commonly called hunting. Specifically, even if the actual suction pressure Ps is falling due to a decrease in the thermal load, for example, an increase in the crank pressure Pc is slow when the Pc/Ps gain is small. Therefore, the displacement does not fall rapidly, and the large-displacement continues. As a result, the actual suction pressure Ps continues falling and overshoots the target suction pressure Pset. The same is true of the case where the suction pressure Ps is increasing due to an increase in the thermal load. With a small Pc/Ps gain, hunting of the suction pressure Ps occurs, particularly when the rotational speed of the swash plate is relatively slow.
To increase the Pc/Ps gain, a difference xcex94Q of the flow rate of the gas that passes through the valve hole should be increased at the time the valve body moves in response to a change xcex94Ps in the suction pressure Ps. That is, the flow rate of the gas should be increased at once when the valve body is moved away from the valve seat. There are two ways to accomplish it as follows.
First, the amount of the displacement of the valve body with respect to a change xcex94Ps in the suction pressure Ps may be increased. In other words, a bellows that produces a large displacement in response to a slight change in the suction pressure Ps can be used. The large displacement of the valve body increases the difference xcex94Q of the flow rate of the gas. However, such a bellows is generally large. Further, the displacement control valve of a variable target suction pressure type compressor requires that the electromagnetic actuator be enlarged in accordance with an increase in the size of the bellows. This leads to a cost increase.
The second way is to enlarge the area of the aperture of the valve hole (the area to be sealed by the valve body). When the area of the aperture of the valve hole is large, the amount of gas that passes through the valve hole changes significantly even if the displacement of the valve body is slight with respect to a change xcex94Ps in the suction pressure Ps.
The larger the aperture of the valve hole is, however, the smaller the inclination of the left portion of the characteristic curve becomes as shown in FIG. 7. In other words, the Pc/Ps gain becomes smaller when the aperture increases. When the aperture of the valve hole is very small (e.g., as in the case xcfx861), the characteristic curve has a steep left portion but the radius of the curve increases gentle in the vicinity of the minimum point, making the Pc/Ps gain smaller. To keep a stable and large Pc/Ps gain over a wide range, it is essential to select the characteristic curve xcfx863 or xcfx864 of the control valve.
The Pc/Ps gain is influenced by the force that act on the valve body, which is based on the differential pressure between the crank pressure Pc and suction pressure Ps. This force is expressed by (Pcxe2x88x92Ps)xc3x97S where S is the aperture area of the valve hole (i.e., S is the effective pressure receiving area of the valve body). The direction of the force is the direction in which the valve body is separated from the valve seat. The larger the aperture area S of the valve hole becomes, the more difficult it becomes for the valve body to be seated due to the force of the differential pressure. When the aperture area of the valve hole is large, therefore, the differential pressure (Pcxe2x88x92Ps) makes it hard for the control valve to be closed. This results in a slow increase in the crank pressure Pc so that the Pc/Ps gain drops.
Accordingly, it is an object of the present invention to provide a control valve for a variable displacement type compressor that can quickly change the crank pressure Pc.
To achieve the above object, the present invention provides a control valve. A control valve controls the displacement of a variable displacement type compressor. The compressor includes a crank chamber, a suction pressure zone, the pressure of which is suction pressure, a discharge pressure zone, the pressure of which is discharge pressure. A bleed passage releases gas from the crank chamber to the suction pressure zone. A supply passage supplies gas from the discharge pressure zone to the crank chamber. The control valve comprises a valve housing. A supply side valve controls the opening degree of the supply passage. A transmission rod extends in the valve housing. The transmission rod moves axially and has a distal end portion and a proximal end portion. A relief side valve control the opening degree of the bleed passage. The transmission rod connects the relief side valve with the supply valve. The relief side valve includes a passage chamber constituting part of the bleed passage. A valve seat defines part of the passage chamber. A relief side valve body contacts the valve seat. The relief side valve body is located in the passage chamber. When the relief side valve body contacts the valve seat, the passage chamber is separated into a first area, which is connected to the crank chamber via an upstream part of the bleed passage, and a second area, which is connected to the suction pressure zone via a downstream part of the bleed passage. A pressure sensing member is located in the first area and moving the relief side valve body in accordance with the pressure in the first area. When the relief side valve body contacts the valve seat, the effective pressure receiving area of the pressure sensing member is substantially equal to the cross sectional area of the passage chamber that is sealed by the relief side valve body.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.